Cellular communication systems ring called parties by sending ring-alert messages on a cell's broadcast channels. Ring-alert messages are directed to particular subscriber units. Subscriber units monitor broadcast channels to determine when a ring-alert message is directed to a particular subscriber unit.
In cellular systems, ring-alerts are generally broadcasted in all cells of an entire geographic region where the called-party may reside. Broadcasting each ring-alert within each cell requires a certain amount of additional energy. In terrestrial cellular systems, there is no need to conserve energy because the energy supply is virtually unlimited.
However, in a satellite communication system, where subscriber units receive ring-alerts from satellite communication nodes, it is desirable to conserve energy by broadcasting ring-alerts in as few cells or antenna beams as possible, since a satellite's energy supply is extremely limited. A satellite's power is generally limited to that supplied by its solar panels and its batteries. Energy expended for ring-alerts reduces the amount of energy available for other communications.
A global satellite telecommunication system, to establish a wireless communication link with subscribers located anywhere in the world, the antennas on the satellites should cover the entire earth with sufficient antenna gain to establish the communication link. When contact is desired with a subscriber unit, the telecommunication system sends the subscriber the ring alert message to alert him that he has an incoming call. If the ring alert message is sent out over all antenna beams in the system, then the message is almost guaranteed to reach the subscriber, since the system antenna footprints cover the entire earth's surface. Sending the message in all antenna beams, however, makes inefficient use of system resources such as the bandwidth available for the ring alert messages, in addition to power discussed above.
Furthermore, in non-geo-synchronous multi-beam satellite communication systems, is extremely difficult to send ring alerts to a large number of mobile subscriber units because the location of a satellite's antenna beams is continually changing with respect to the earth's surface. This is true even though the locations of the mobile subscriber units can be provided to the satellite. This problem increases substantially when the satellites are in a low-earth orbit (LEO) because the relative velocity between the satellite and the subscriber unit is significant.
Ring-alerts and pages are desirably directed within the proper antenna beam of the proper satellite in a time-efficient manner to provide satisfactory service to users placing calls, and to conserve satellite energy.
Thus it is desirable to send ring alert messages to a smaller subset of the system antenna beams that the ring alert message can be sent to conserve bandwidth resources. It is also desirable to send ring alert messages to a smaller subset of system antenna beams with a high probability of contacting the subscriber unit.
Thus what is needed is an improved method of sending a ring alert to a subscriber unit. What is also needed is method of sending ring alerts to subscriber units that reduces processing power and conserves energy a the subscriber unit. What is also needed is a method of sending ring alerts to subscriber units that reduces the number of registrations performed by a subscriber unit. What is also needed is a method of sending ring alerts to a subscriber unit that improves likelihood that the subscriber unit will receive the ring alert. What is also needed is method of sending ring alerts to subscriber units that reduces use of system bandwidth while having a higher probability of reaching the subscriber unit.